In a copier, facsimile, printer, or other image forming apparatus using the electrophotographic process, the electrostatic latent image formed on the photosensitive body is converted to a visible image by a developer (hereinafter referred to as a “toner”). A toner is mainly comprised of a binder resin in which colored particles comprised of a colorant and, in accordance with need, a charge control agent, parting agent, etc. are dispersed.
Toners are roughly classified by process of production into pulverized toners comprised of colored particles obtained by the pulverizing method and polymerized toners giving colored particles by the polymerization method (in the present invention, colored particles obtained by the polymerization method are referred to as “colored polymer particles”). With the pulverizing method, a binder resin comprised of a thermoplastic resin is melt kneaded with a colorant, charge control agent, parting agent, and other additive ingredients, pulverized, and classified to obtain colored particles and produce the pulverized toner. As opposed to this, with the polymerization method, a polymerizable monomer composition containing a polymerizable monomer, colorant, and other additives is used to form droplets in an aqueous dispersion medium which are then polymerized to produce colored polymer particles forming the polymerized toner.
Recently, color copiers, printers, and other image forming apparatuses using the electrophotographic process are rapidly spreading. In color image formation, photographs and other high definition images are also being formed, so image formation with a particularly high resolution and good color reproducibility is being sought. High quality color toners able to meet with this demand are considered necessary. From this viewpoint, spherical toners are suitable in that they are good in transferability and dot reproducibility. The suspension polymerization method, dispersion polymerization method, emulsion polymerization method, and other polymerization methods enable spherical toners to be produced with a good efficiency, so toners made by the polymerization method (polymerized toners) are becoming the mainstream.
On the other hand, recently, environmental regulations are being toughened. In such image forming apparatuses as well, therefore, the low molecular weight ingredients which volatilize when fixing a toner by heat such as the polymerizable monomer remaining in the toner (in the present invention, hereinafter referred to as the “residual monomer”) and other volatile organic compounds (in the present invention, hereinafter referred to as the “residual VOC”) are becoming an issue. Further, if the toner contains a large amount of residual monomer or residual VOC, offset easily occurs, a film of the toner easily forms on the surface of parts of the image forming apparatus such as the photosensitive body and development blade, and other problems arise.
Therefore, to produce toners with little residual monomer and residual VOC, in the past methods have been proposed for removing the residual monomer and residual VOC from the colored polymer particles after polymerization. Removal of the residual monomer and residual VOC is more difficult with toners made by the polymerization method than with toners made by the pulverizing method. That is, with the pulverizing method, it is possible to remove the residual monomer and residual VOC using means such as heat treatment of only the binder resin before the colored particles are formed. As opposed to this, with the polymerization method, the colored polymer particles (colored particles) are formed simultaneously with the polymerization, therefore this removal step must be performed after the formation of the colored particles. For this reason, the residual monomer etc. are easily absorbed in the ingredients other than the binder resin (colorant, charge control agent, parting agent, etc.) and therefore are difficult to remove compared with the case of only a binder resin. Further, if overly heating by a long time or high temperature, the colored polymer particles aggregate, the other ingredients besides the binder resin in the colored polymer particles become degraded, and for other reasons, the obtained toner easily declines in quality.
In recent years, there have been increasing demands for lowering the lowest fixing temperature so as to increase the speed of printing, save power, etc. Further, with full color printing, four colors are superposed, so a high print durability of the different colors of toners even in different environments is being sought. In such toners, it has been extremely difficult to remove the residual monomer and residual VOC without aggregation of the colored polymer particles, degradation of the ingredients in the colored polymer particles, etc. causing a drop in the toner quality.
As the stripping method for removing residual monomer and residual VOC from the aqueous dispersion medium containing colored polymer particles obtained by the polymerization method, various methods have been proposed in the past.
Japanese Patent Application Laid-Open No. 5-100485 discloses the method of steam stripping comprising blowing saturated steam into the aqueous dispersion medium containing colored polymer particles obtained by the polymerization method. However, with this method, there was the problem that the shear force when the saturated steam came into contact with the colored polymer particles easily led to agglomeration of the colored polymer particles.
Further, Japanese Patent Application Laid-Open No. 2001-92180 discloses a process of production of a toner comprising running a relatively large amount of nitrogen gas through the vapor phase of a distillation system containing the aqueous dispersion medium containing colored polymer particles obtained by the polymerization method and reducing the pressure for the purpose of stripping. However, even if using this method, the efficiency of removal of the residual monomer etc. is insufficient. Further, there was the problem that the obtained polymerized toner ends up deteriorating in print durability. Further, a large amount of nitrogen or other gas is used, so the energy consumption ends up increasing.
Further, Japanese Patent Application Laid-Open No. 2004-271816 proposes a process of production of a toner comprising injecting nitrogen gas at an normal pressure of by 101 kPa into an aqueous dispersion medium containing colored polymer particles obtained by the polymerization method for the purpose of stripping. According to this method, it was possible to efficiently remove the residual monomer, but there was the problem that the efficiency of removal of the residual VOC was insufficient and if increasing the flow rate of the nitrogen for improving the efficiency, a large amount of bubbles would form in the aqueous dispersion medium making operation impossible.